The government's master watch list is one of roughly a dozen lists, or databases, used by counterterrorism officials. Officials have periodically adjusted the criteria used to maintain it.

But civil liberties groups argue that the government's new criteria, which went into effect over the summer, have made it even more likely that individuals who pose no threat will be swept up in the nation's security apparatus, leading to potential violations of their privacy and making it difficult for them to travel.

"They are secret lists with no way for people to petition to get off or even to know if they're on," said Chris Calabrese, legislative counsel for the American Civil Liberties Union.

Officials insist they have been vigilant about keeping law-abiding people off the master list. The new criteria have led to only modest growth in the list, which stands at 440,000 people, about 5 percent larger than last year. The vast majority are non-U.S. citizens.